In recent years, flat-panel displays have been used in many fields and places, and the importance thereof has been increasingly becoming higher while informatization has been advancing.
It is said that the current major presence of flat-panel displays is liquid crystal displays (LCDs). Active developments have been made also about flat-panel displays based on a display principle different from that of liquid crystal displays (LCDs), such as organic ELs, inorganic ELs, plasma display panels (PDPs), light emitting diode display devices (LEDs), vacuum fluorescent display devices (VFDs), and field emission displays (FEDs).
These new flat-panel displays are each called a self-luminous type display. The displays are largely different from liquid crystal displays (LCDs) in points described below, and have excellent features that no liquid crystal displays (LCDs) have.
Liquid crystal displays (LCDs) are each called a light-receiving type display. Their liquid crystal itself does not emit light, and operates as the so-called shutter, which transmits and blocks external light, to constitute a display device. Accordingly, a light source is required, and a backlight is generally required. By contrast, about the self-luminous type display, a device thereof itself emits light so that no additional light source is required. Moreover, in the light-receiving type display, such as a liquid crystal display (LCD), a backlight is constantly lighted in spite of the manner of information displayed thereon, so that the display constantly consumes an electric power not different from that consumed in a full-display state. By contrast, in the self-luminous type display, only its part required to be lighted on correspondingly to information to be displayed consumes electric power; thus, the display in principle has an advantage of being smaller in power consumption than any light-receiving type display device.
Similarly, liquid crystal displays (LCDs) block light from their backlight source to gain a dark state; thus, even when the light is small in quantity, it is difficult that a leakage of the light is completely prevented. By contrast, in the self-luminous type display, the state that the display emits no light is exactly a dark state, so that the display can easily gain an ideal dark state. Thus, the self-luminous type display is overwhelmingly advantageous over the light-receiving type in contrast also. Furthermore, liquid crystal displays (LCDs) make use of polarization control based on the birefringence of liquid crystal, so as to be large in the so-called viewing angle dependency, which is a property that the display state of the displays is largely varied in accordance with the observation direction thereof. However, in the self-luminous type display, this problem is hardly caused. Additionally, liquid crystal displays (LCDs) make use of an alignment change originating from the dielectric property anisotropy of liquid crystal, which is an organic elastic material; thus, the response time thereof to an electrical signal is 1 ms or more in principle. By contrast, the above-mentioned self-luminous type techniques, the development of which has been advanced, make use of, for example, the so-called carrier transition, such as electron/hole transition, electron discharge, or plasma discharge; thus, the response time thereof is in the order of ns. In short, the response thereof is overwhelmingly speedier than that of liquid crystal techniques, so that the self-luminous type techniques do not have a problem of residual moving-images, which originate from response lags of liquid crystal displays (LCDs).
Recently, among these display devices, particularly, organic EL display devices have been actively researched. For example, the following have been suggested: (1) a display device in which light emitting layers in the three original colors are formed into predetermined patterns which are arranged in accordance with the respective emitted light colors; (2) a display device in which a light emitting layer which can emit white light is used to emit the light through a color filter in the three original colors, so that displays are attained; (3) a display device in which a light emitting layer which can emit blue light and a color conversion layer using a fluorescent dye is laid, whereby the blue light is converted to green fluorescent light or red fluorescent light so that displays in the three original colors are attained.
Organic EL display devices have been made into practical use mainly for small screens having a size of several inches in, for example, vehicle-mounted navigation systems, portable telephones or digital cameras. Recently, for flat-panel articles, typical examples thereof being flat-panel televisions, their screens have been required to be made as large as, for example, 20 inches or more in size.
However, when an organic EL display device is formed to have a large screen having a size of, for example, 20 inches or more, wirings through which driving current is supplied to its individual pixels are extremely different in length between the outer circumferential region and the central region of the screen. Thus, in the screen central region, the degree of a voltage fall is larger than that in the screen outer circumferential region to cause an inconvenience that an unevenness in the brightness of the screen is generated. This tendency becomes more remarkable as the screen is intended to be made larger. In particular, to the wirings through which driving voltage is supplied, electric current is sent through a transparent electrode made of, e.g., ITO that is formed on the organic EL layer containing a light emitting layer. This material, ITO, is larger in electrical resistance than, for example, metallic Cu, so that a voltage fall based on ITO produces a large effect.
Incidentally, the driving manner of organic EL display devices is classified into the passive matrix manner and the active matrix manner. According to the former, the EL display devices are simple in structure, but there is caused a problem that display devices which are large and give highly minute images are not easily realized. In order to make the EL display device large, the latter manner, the active matrix manner, has been actively developed. According to the active matrix manner, electric currents flowing organic EL elements arranged in respective pixels are controlled by TFTs (thin film transistors) fitted to the organic EL elements, respectively, inside a driving circuit.
Incidentally, examples of prior art appear to be related to the present invention include the following Patent Literatures 1 to 4: